svn commit: r363625 - stable/12/usr.sbin/mountd

[Rick Macklem]

Brooks Davis wrote:
>On Thu, Jul 30, 2020 at 03:48:34PM +0000, Rick Macklem wrote:
>> Rick Macklem wrote:
>> >Ian Lepore wrote:
>> >>On Thu, 2020-07-30 at 01:52 +0000, Rick Macklem wrote:
>> >>> Brooks Davis wrote:
>> >>> > Author: brooks
>> >>> > Date: Mon Jul 27 23:18:14 2020
>> >>> > New Revision: 363625
>> >>> > URL: https://svnweb.freebsd.org/changeset/base/363625
>> >>> >
>> >>> > Log:
>> >>> >  MFC r363439:
>> >>> >
>> >>> >  Correct a type-mismatch between xdr_long and the variable "bad".
>> >>> >
>> >>> > [...]
>> >>> --> I can't see how the xdr.c code would work for a machine that is
>> >>> BIG_ENDIAN and where "long" is 64bits, but we don't have any of
>> >>> those.
>> >>>
>> >>
>> >>mips64 and powerpc64 are both big endian with 64-bit long.
>> >Oops, I didn't know that. In the past, I've run PowerPC and MIPS, but thought
>> >they both were little endian. (I recall the arches can be run either way.)
>> >
>> >Anyhow, take a look at head/lib/libc/xdr/xdr.c and it looks to me like it
>> >has been broken "forever" (ever since we stopped using a K&R compiler
>> >that would have always made "long" 32bits).
>> OK, I took another look at xdr.c and it isn't broken as I thought.
>>
>> xdr_long() takes a "long *" argument ("long" in Sun XDR is 32bits),
>> but then it only passes it as an argument to XDR_PUTLONG(), which is actually
>> a call to xdrmem_putlong_aligned() or xdrmem_putlong_unaligned().
>> For xdrmem_putlong_aligned(), the line is:
>>        *(u_int32_t *)xdrs->x_private = htonl((u_int32_t)*lp);
>>       --> where lp is a "long *"
>>
>> I'll admit I'm not 100% sure if "(u_int32_t)*lp" gets the correct 32bits of a 64bit
>> long pointer for all arches? (I'm not very good at knowing what type casts do.)
>> If this is the equivalent of "u_int32_t t; t = *lp; htonl(t); then I think the code is ok?
>> (At least it makes it clear that it is using 32bits of the value pointed to by the
>>  argument.)
>>
>> For xdrmem_putlong_unaligned(), it does the same thing via:
>>         u_int32_t l;
>>         ?.
>>         l = htonl((u_int32_t)*lp);
>>
>> --> At least the man page for xdr_long() should be clarified to note it
>>       puts a 32bit quantity on the wire.
I think I will try and come up with a man page patch, noting that xdr_long()
always puts 32bits on the wire, even if long is 64bits for the arch.

>>
>> >If anyone has either of these and can set up an NFS server on one of
>> >them and then try and do an NFSv3 mount that is not allowed, it would
>> >be interesting to see the packet trace and if the MNT RPC fails, because
>> >it looks like it will put the high order 32bits on the wire and they'll
>> >always be 0?
>> It would still be interesting to test this on a 64bit big endian, but so long as
>> the above cast works, it does look like it works for all arches.
>>
>> >Just to clarify. The behaviour wasn't broken by this commit. I just
>> >don't see how the commit fixes anything?
>> My mistake. Sorry for the noise.
>>
>> I now think the commit is correct since it uses "*lp" to get the value before
>> casting it down to 32bits. Passing in an "int *" was incorrect.
>>
>> The code does seem to handle "long *" for 64bit arches, although it
>> only puts 32bits "on-the-wire".
>>
>> rick, who was confused because he knew there was only supposed to be
>>         32bits go on the wire.
>
>Thank you for all the analysis.  I'd initially changed all the uses
>of bad to use xdr_int(), but switched to this "fix" because it's what
>NetBSD and OpenBSD have been using for over a decade (and there was
>less churn).  I'm happy to flip it the other way if that seems more
>correct/less confusing.
I think your current patch is fine. The confusion is w.r.t. what xdr_long() does
for a 64bit long and I think a man page update may be the way to go.
--> If you look in xdr.c, xdr_int() assigns the value to a long and then ends
      up truncating it back down, similar to xdr_long().
      --> Some of the stuff in xdr.c is pretty scary for 64bit longs, but it all
             seems to work, once you look at it for a while.;-)

>The previous code does in fact cause a 64-bit load of a pointer to an
>int on 64-bit platforms.  I hit this in CheriBSD because that pointer
>had 4-byte bounds.
Yes. The first time I looked at the code (it was late evening), I misread
    ((u_int32_t)*lp)  as *((u_int32_t *)lp) and that was why I thought your patch
    was broken.

Thanks for doing this and sorry about the noise, rick
ps: Personally, I've never understood why ANSI C allowed "long" to be 64bits
     on some arches. I still bump into hassles because the old K&R code was
     written assuming long to be 32bits.

-- Brooks